1. Field of the Invention
The present invention relates to an anti-howling circuit useful for suppressing howling in a system including an adaptive echo canceler.
2. Description of the Related Art
Adaptive echo cancelers are widely used in telecommunication systems to cancel line echo and acoustic echo. Line echo is generated in the hybrid circuits that interface between two-wire and four-wire signal channels; acoustic echo is generated when the acoustic signal from a loudspeaker is picked up by a microphone. An adaptive echo canceler is a computing device based on an adaptive filter. Adaptive echo cancelers may be installed in telephone sets or telephone switching apparatus.
Adaptive echo cancelers are not always able to follow the rapid changes in the echo path that occur when a call is transferred from one extension line to another, or when a telephone is used in the hands-free mode. Uncanceled or incorrectly canceled echo may then circulate through the telephone network, building up into a howling tone. To prevent this, echo cancelers also detect and suppress howling. In a conventional howling detector of the type described in, for example, Japanese Unexamined Patent Application Publication No. 06-044724, howling is detected on the basis of the concentration of the energy of the microphone signal into a narrow signal band.
A problem with this detection system is that such concentration of signal energy does not always imply howling. Occasionally the energy of a natural speech signal is likewise concentrated into a narrow band. If a howling suppressor reduces the gain in, for example, the automatic gain control system of a loudspeaker amplifier on the basis of howling detection by this method, although howling is suppressed, desired speech is also sometimes suppressed, degrading the quality of the speech signal.